Abstract : A study was made of the influence of higher-order and nonlinear effects on the free vibration behavior of thin circular cylindrical shells. A recent solution utilizing the Karman-Donnell strain-displacement relations is examined and criticized. A new solution is carried out which removes the basis for criticism and discloses the existence of a nonperiodic vibration behavior, a phenomenon heretofore unknown. Further, solutions are obtained employing the strain-displacement relations deduced by Sanders and applied, in an appropriately modified manner, by Mayers and Rehfield to the shell postbuckling problem. The effect of the improved strain-displacement relations in predicting the vibration behavior, attendant to a modal shape possessing a small number of circumferential waves, is assessed and recommendations are made for directing future effort on the problem. (Author)